Dark field illuminator



May 9, 1939.

R. M. SHIPLEY, JR

DARK FIELD ILLUMINATOR Filed Jan. 25, 1938 Patented May 9, 1939 UNITED STATES PATENT OFF'ICE 3 Claims.-

This invention relates to instruments known as dark field illuminators. These instruments are used principally for observing the perfections, or imperfections in precious stones, and are constructed so that light is concentrated upon the gem from the sides with a dark field, or backround a ainst which the gem is examined.

The invention relates particularly to an instrument of this type employing a primary, or basal, reflector and a secondary, or upper, reflector, and in which the source of illumination, or lamp, is located at about the focus of the basal reflector. The upper reflector is in the form of an annular shell, the upper portion of which is removed; that is to say, the portion which would cover its focal point.

The general object of this invention is to provide a relation of the reflecting surfaces, which will operate to effect a high lateral illumination of the gem being examined, but which, at the same time, will reduce the glare effect upon the eye of the expert who is examining the gem, to a minimum.

Further objects of the invention will appear hereinafter.

The invention consists in the novel "parts and combination of parts to be described hereinafter, all of which contribute to produce an efllcient dark fleld illuminator.

A preferred embodiment of the invention is described in the following specification, while the broad scope of the invention is pointed out in the appended claims.

In the drawing, Fig. 1 is a vertical section through a dark field illuminator embodying my invention, the lamp socket being shown partially in elevation.

Fig. 2 is a plan of a slide employed with the device.

In practicing the invention, I provide a casing I, preferably in the form of a cylindrical shell open below and having means such as an inturned flange 2 at its upper edge, so that an opening 3 of restricted diameter is formed at the upper side of the instrument. The lower portion of the casing I may be supported on a suitable stand 4. At the lower portion of the casing I I provide a primary reflector, or basal reflector i, if the instrument is constructed on a vertical axis, and the base of the primary reflector is provided with a pocket 6 to support an illuminating device such as an electric lamp 1, which lamp can be screwed into a socket 8 received in the pocket 6, the current to the lamp being received through an elec- 55 tric cord attached to a suitable electric connection or coupling member 9. The primary reflector 5 is preferably a parabolic reflector, and the lamp 1 is located so that its filament will lie substantially at the focal point In of the reflector. The primary reflector 5 may be supported in the 5 casing I in any suitable manner, for example, by providing a marginal flange I I at the upper edge of the reflector that seats upon a ring or rest l2 attached to the inner face of the casing.

Above the primary reflector 5 I provide a secondary reflector I3, which is also preferably of parabolic form, but truncated, that is to say, the upper portion which would surround its focus, is cut away so that the reflector is annular and the upper end of this reflector presents a large opening I4, which is preferably located within the opening 3, the latter opening being of slightly larger diameter.

The gem, for example, the diamond I5 to be examined, is held at the focal point of the secondary reflector I 3 in any suitable manner. In practice, the means for this purpose usually involves a suitable clamp I6 supported on an arm ll that is supported at the side of the instru- -ment on a suitable post I8 on which the arm ll pivots. The secondary reflector I3 may be supported in any suitable manner within the casing I, for example, by resting upon radial arms I9. These arms are in the form of plates, the inner ends of which are rigidly secured to the side walls of a circular box or rest 24, to be described more fully hereinafter. A follower ring 20 may be employed to hold the flange in position.

With this arrangement it will be evident that the rays of light from the focal point It will be reflected upwardly around the periphery of a dark background 220. as indicated by the dotted lines 2|, so that the rays are concentrated around the superficial area of the diamond supported at the upper focal point. 40

The dark field, or background, against which the diamond is viewed, is provided preferably by means. of a slide 22 that slides in through the casing I and the reflector I3 through slots 23 and 23a. This slide 22 is located over a circular 5 box or rest 24, the axis of which substantially coincides with the line joining the focal points of the reflectors. The slide 22 may be reversible, one-half of it being formed of an opaque plate 22a, and the other half composed of a translucent plate 22b. The rest 24 may have a central opening 25 formed with an inturned flange 26, said flange being formed with a stop '21 for the forward edge of the slide. The arms I9 support the rest 24.

In order to prevent the glare from the reflecting surface of the secondary reflector II from detracting from the perfect vision of the observer in examining the diamond, the reflecting surface l3a of the secondary reflecto is formed or treated so that it has a partialdifl'using effect on the light rays reflected by it upon the diamond. This is accomplished by employing a flnish which is known as a "satin finish. By this means although the diamond will be highly illuminated, its capacity for enabling imperfections in the diamond to be observed is greatly increased.

The satin finish on the secondary reflector is highly advantageous for the reason that it permits the observer's eye to examine the diamond without being blinded by too intense reflected rays. However, a quite beneficial effect can be obtained by providing the satin finish on the reflecting surface of the primary reflector, and using the usual highly polished reflecting surface for the secondary reflector. In accordance with my invention, one or the other of these reflectors, however, should have the diffusing effect.

In examining some stones for other purposes, it may be desired to employ the instrument without the dark background, and with the under face of the diamond illuminated by direct rays passing up from the lamp, but slightly diffused. In order to permit this instrument to be used in this way, the slide 22 should be removed and reversed so as to present the translucent plate 22b between the gem and the lamp. Of course, such translucent plates or screens may have different degrees of translucency so as to enable the amount of transmitted light to be regulated as desired.

What I claim is:

1. A dark field illuminator for examining precious stones, having a primary substantially parabolic reflector and a secondary substantially parabolic reflector, said reflectors being mounted substantially coaxially with their concave sides toward each other, a source of illumination located substantially at the focal point of the primary reflector, the secondary reflector being truncated and of annular form so as to present an opening, and with its focal point located beyond the plane of the opening with respect to the body of the reflector, one of said reflectors having a surface finish operating to partially diffuse the reflected light; and a dark background located on the axes of the reflectors ,and

between the reflectors against which the stone may be viewed, said reflectors cooperating to reflect the rays from the source of illumination, around the periphery of the background and up through the said opening onto the stone when held at the focal point of the secondary reflector.

2. A dark fleld illuminator for examining precious stones, having a primary substantially parabolic reflector and a secondary substantially parabolic reflector, said reflectors being mounted substantially coaxially with their concave sides toward each other, a source of illumination located substantially at the focal point of the primary reflector, the secondary reflector being truncated and of annular form so as to present an opening, and with its focal point located beyond the plane of the opening with respect to the body of the reflector, said secondary reflector having a surface flnish operating to partially diffuse the reflected light; and a dark background located on the axes of the reflectors and between the reflectors, against which the stone may be viewed, said reflectors cooperating to reflect the rays from the source of illumination, around the periphery of the background and up through the said opening onto the stone when held at the focal point of the secondary reflector.

3. A dark field illuminator for examining precious stones, having a casing disposed on a vertical axis with an opening at its upper end, a primary reflector supported in the lower portion of the casing, a lamp carried at the focus of the primary reflector, a secondary reflector consisting of a truncated annular shell presenting an opening at its smaller end and with its focal point located beyond the plane of the opening with respect to the body of the shell, said reflectors being disposed coaxially with their concave sides toward each other, said secondary reflector having a slot in the wall thereof, a slide adapted to he slid through said slot and carrying a dark background on its upper side located on the axes ofthe reflectors, said lamp and reflectors cooperating to reflect the rays upwardly around the periphery of the dark fleld and up through said opening to the focal point of the secondary reflector to cast the reflected rays onto the precious stone when held at the focal point of the secondary reflector and viewed against the said background.

ROBERT M. SHIPLEY, JR. 

